In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Photopolymers can be sensitized for blue, green or red light (i.e. wavelength range between 450 and 750 nm), for violet light (i.e. wavelength range between 350 and 450 nm) or for infrared light (i.e. wavelength range between 750 and 1500 nm). Laser sources have been increasingly used to expose a printing plate precursor which is sensitized to a corresponding laser wavelength. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material. An infrared laser diode emitting around 830 nm or a Nd-YAG laser emitting around 1060 nm can also be used.
Typically, a photopolymer plate is processed in alkaline developer having a pH>10. Currently, most commercial lithographic plates require an additional gumming process after the exposed plates is developed and before it is put on the press, in order to protect the plate from contamination, e.g. by oxidation, fingerprints, fats, oil or dust, or from damaging, e.g. by scratches during handling of the plate. Such an additional gumming step is not convenient for the end-user, because it is a time consuming step and requires an additional gumming station.
WO 02/101 469 discloses a method of processing an imagable element useful as alkaline-developable lithographic printing plate precursor wherein the element is developed and gummed with an aqueous alkaline developing-gumming solution comprising a water-soluble polyhydroxy compound having a specific structure.
EP 1 342 568 discloses a method for making a heat-sensitive lithographic printing plate wherein the image-wise heated precursor, comprising a coating of hydrophobic thermoplastic polymer particles which coalescence on heating, is developed with a gum solution. A practical embodiment for this type of printing plates was introduced by Agfa under the tradename Azura.
WO 2005/111727 discloses a method for making a lithographic printing plate wherein the image-wise exposed precursor, comprising a photopolymerisable coating, is developed with a gumming solution.
In WO 2006/026229, EP 1 602 982, EP 1 621 339, EP 1 630 618, EP 1 705 522, EP 1 755 002 and US 2007/0020563, a method is disclosed wherein an image-wise exposed photopolymer printing plate precursor is developed with an aqueous solution.
In U.S. Pat. No. 6,027,857, U.S. Pat. No. 6,171,735, U.S. Pat. No. 6,420,089, U.S. Pat. No. 6,071,675, U.S. Pat. No. 6,245,481, U.S. Pat. No. 6,387,595, U.S. Pat. No. 6,482,571, U.S. Pat. No. 6,576,401, WO 93/05446, WO 03/087939, EP 1 518 672, EP 1 562 078, EP 1 577 090, US 2003/16577, US 2004/13968, WO 2005/111717 and WO 2006/007270 an alternative method is disclosed for preparing a lithographic printing plate wherein a photopolymer plate, after image-wise exposure, is mounted on a press and processed on-press by applying ink and fountain to remove the unexposed areas from the support.
A problem associated with the lithographic printing plate precursors of the prior art, after image-wise exposure, which are processed on-press with fountain and ink or which are processed and gummed in a single step by using a gumming solution, is an insufficient roll-up performance. This means that the ink acceptance at the printing areas, i.e. the exposed areas, is too weak during the start-up of the printing process, resulting in a too low take-up of ink at the printing areas and a too low optical density on the first number of printed sheets. It is desirable that this number of these “unusable” printed sheets is as low as possible in order to reduce the waste of paper in the start-up of the printing process.